Collect all 64 Cubes, then go through the Star Gate again in the 32 Cube room for the alternate ending. After the credits end, select the "New Game +" option at the main menu. Progress to where you originally obtained the Fez and sunglasses to instead get a pair of red and blue 3D glasses. They will enable Stereoscopic mode. To disable this mode, enter the "Settings" menu.

Sunglasses (first person view)

Start a game in New Game + mode, and progress to where you originally obtained the Fez to instead get a pair of sunglasses. Click the Left Analog-stick to switch to first person view.

Hidden Black Monolith text

Start a game in New Game + mode, and enable the "Flight mode" code when meeting the Hexahedron. You can move as it, and once exiting the screen from successfully completing the view rotation tutorial it provides, you will be taken to another screen with the Black Monolith in first-person. The text, translated, is: